System for improved reproduction of sound



April 16, 1968 B. J. EISEMAN, JR

SYSTEM FOR IMPROVED REPRODUCTION OF SOUND Filed March 22, 1966 2Sheets-Sheet 1 w FIG. 2 5 k 1 l HI 3 I PERFLUORQCYCLOBUTANE & AIR :50: ff. 515 K LL-l 1 A I :70

g 2 5 456789l 2 5 4 56789| 2 3 456789| w 20 I00 I000 |0,000 FREQUHICY INCYCLES PER SECOND INVENTOR BERNHARDT EISEMAN, JR.

ATTORNEY IN 0 E C I BE L S S 0 U N 0 L E V E L April 16, 1968 B. .I.EISEMAN, JR 3,378,098

SYSTEM FOR IMPROVED REPRODUCTION OF SOUND Filed March 22, 1966 2Sheets-$heef 2 FIG-3 90 L l I I I AIR 0 NLY FIG. 4 T I I I I I I I I'PERFLUOROCYCLOBUTANE ONLY 8 FIG-5 I I I I I I I I- PERFLUOROCYCLOBUTANEONLY I WITH POLYESTER FILN as 2 3 4 5 6 7 B 9 I 2 5 4 5 6 7 8 9| 2 I 4 56. 7 8 9 I 20 I00 I000 l0.000 FREQUEN CY IN CYCLES PER SECOND INVENTORW2. 6mm.

ATTORNEY United States Patent 3,378,098 SYSTEM FOR IMPROVED REPRODUCTIONOF SOUND Bernhardt Joseph Eiseman, Jr., Wilmington, Dcl., assignor to E.I. du Pont de Nemours and Company, Wilmington, Del., a corporation ofDelaware Filed Mar. 22, 1966, Ser. No. 536,411 11 Claims. (Cl. 181-31)This invention relates to an improved loudspeaker system for thereproduction of sound and, more particularly, to a loudspeaker system inwhich the speaker diaphragm is in contact with a po-lyfluorinatedcompound.

In trying to reproduce instrumental, vocal and other sound via radio andrecordings, deficiences are encountered in the ability to reproducefaithfully the original sound. In the better reproduction systems, themost perceptible deviations from the original sound can generally beattributed to deficiencies in the ability of the loudspeaker system torespond accurately to the impulses being supplied to it. Thesedeficiencies are most often due to a nonuniformity in the intensity ofresponse at different frequencies.

As a general rule, most speakers do not respond to impulses at differentfrequencies with equal intensity. Most good speakers respond with fairlyuniform intensity to the middle frequencies of about 300 to 3,000 cyclesper second, but substantial reductions in intensity are encountered withimpulses of higher and lower frequencies.

Losses in intensity at the lower frequencies of about 50-300 cycles persecond can be compensated for to some extent by carefully selecting theenclosure used with the speaker. Since a peak of sound intensitygenerally occurs at the resonant frequency of the enclosure, theseintensity losses can be reduced to some extent by using an enclosurehaving a resonant frequency in the range of the intensity losses. Theresonant frequency of the enclosure is shifted to these lowerfrequencies by increasing its size. In order to provide a resonantfrequency in the range of 50 to 300 cycles per second, an enclosure sizeof the order of about 5 cubic feet or more is required. However, sincethe emphasis today is in the direction of reducing the size of speakerenclosures to render them more desirable for use in home high-fidelitysystems, such large enclosures are not practical in many instances.

Another method of improving the response of a speaker system at lowfrequencies is to use a high compliance speaker, that is, one having adiaphragm support which is relatively non-rigid, resulting in a lowerspeaker resonance. Although such speakers do have improved bassresponse, they are more expensive and require a considerably greateramount of power, of the order of several times the amount ordinarilyused, to drive them. Moreover, high compliance speakers also introducedistortions of their own, due to their non-rigid construction. Thusthere is no simple and economical method of improving the bass responseof an ordinary speaker.

It is even more difficult to overcome speaker deficiencies in intensityof response to the higher frequencies of about 3,000 to 10,000 cyclesper second and above. These deficiencies can be corrected only bycombining the speaker with one or more additional speakers whichemphasize these higher frequencies.

Another factor which contributes to the quality of reproduction of soundis the clarity and definition of the sound. All speaker systems have atleast minor deficiencies in this regard. Apparently this factor isdetermined to some extent by the damping characteristics of the speakerdiaphragm, although very little is known about its control.

It is an object of this invention to improve the quality of soundreproduced by speakers. Another object is to broaden the frequencyresponse of speakers to include higher and lower frequencies than usual.A further object is to provide a simple means by which a speaker can bemade to respond with more uniform intensity at different frequencies. Astill further object is to improve the clarity and definition of soundemitted by a speaker. These and other objects will become apparent fromthe following description of this invention.

It has now been discovered that the quality of sound reproduced byspeaker systems can be dramatically improved by providing a loudspeakersystem comprising a loudspeaker containing a sound producing diaphragmin which at least one surface of the diaphragm is in contact with agaseous atmosphere of a non-flammable polyfiuorinated aliphatic oralicyclic compound characterized in that r MIT 1 is less than 1.2wherein MW is the molecular weight of the compound, Cp is its specificheat at constant pressure and Cv is its specific heat at constantvolume.

FIGURE 1 illustrates a typical loudspeaker system used in accordancewith this invention. FIGURE 2 is the frequency response curve for aspeaker system in which the front surface of the speaker diaphragm is incontact with air and the back surface is in contact withperfluorocyclobutane. FIGURE 3 is the frequency response curve for aspeaker system in which both surfaces of the speaker diaphragm are incontact with air. FIGURE 4 is the fre quency response curve for aspeaker system in which both surfaces of the speaker diaphragm are incontact with perfiuorocyclobutane. FIGURE 5 is the frequency responsecurve for a speaker system in which both surfaces of the speakerdiaphragm are in contact with perfiuorocyclobutane and a thin polyesterfilm separates the perfluorocyclobutane in front of the diaphragm fromthe atmos-- phere.

Quite surprisingly, the polyfluorinated gases defined above have beenfound to possess an acoustic property which greatly improves the qualityof sound emitted from speaker diaphragms in contact with them. Whenusing these gases in accordance with this invention, the frequency rangeover which the speaker system responds fairly uniformly is broadened toinclude both higher and lower frequencies than normal. Moreover, thespeaker response is more uniform over this broader range than over thenarrower range for air. Furthermore, the sound emitted from the speakerhas improved clarity and definition. The practical result of theseacoustic effects is an improved quality of reproduction of sound withsignificant enhancement of the lis'teners pleasure.

Although it is intended that this invention not be re stricted to anyparticular theory, it is considered a pos sibility that the improvementin frequency response observed in accordance with this invention is dueto the defined gases having an unusual elasticity to sound waves wherebythere is only relatively slow absorption and degradation of sound energyto heat. It is believed that the improved clarity and definition ofsound emitted from speaker systems of this invention may be due in partto improved damping of the speaker diaphragm by the defined gases,perhaps resulting to some extent from their being heavier than air.Thus, the unusual acoustic properties of these gases appear to be due toa unique combination of properties with respect to sound waves.

The polyfluorinated compounds used in accordance with this inventionshould be non-flammable, non-corrosive and non-toxic compounds which arecapable of existing as a gaseous atmosphere and for which is less than1.2. A decrease in the effect would be expected as the value of @n 2 MWCU 1) increases as a function of an increase in the heat capacity ratio(QB Cv In view of this graduated effect, it is preferred that the p 2 Cvvalue be less than 1.0. By polyfiuorinated compounds is meant compoundscontaining three or more florine atoms per molecule. By non-flammable ismeant nonfiammable in air at room temperature. Polyfluorinated aliphaticand alicyclic compounds which have been found to possess the requisitecharacteristics include aliphatic polyfluorohydrocarbons such ashexafiuoroethane, 1,1,1,- 2,2,3,3heptafluoropropane, octafiuoropropaneand decafluorobutane; alicyclic polyfiuorohydrocarbons such asl,3-bis(trifluoromethyl)hexafiuorocyclobutane and octafluorocyclobutane;aliphatic chloropolyfiuorohydrocarbons such as chloropentafluoroethane,1,2-dichlorotetrafiuoroethane, 2-chloroheptafiuoropropane, and1,2-dichlorohexafiuoropropane; and aliphatic polyfiuoroethers such as1,1,1,2-tetrafluoroethyl heptafiuoropropyl ether.

The beneficial acoustic effect derived from using gases of thisinvention is about proportional to the molar concentration of theinvention gas. That is, if the invention gas is diluted with another gasor gases not within the scope of this invention and the total pressureis held constant, the desirable effect is about proportional to the molefraction of the invention gas. Similarly, if the partial pressure of theinvention gas is held constant and another gas is added to increase thetotal pressure, no adverse effect is observed. As little as about byvolume of invention gas is advantageous in admixture With other gases.

The acoustic effect of gases of this invention also varies in proportionto the partial pressure of the gas. When the concentration of theinvention gas is increased by increasing its pressure, a correspondingincrease in its effectiveness is obtained. However, if the saturationpressure is approached or reached, whether or not condensation of liquidactually takes place, the beneficial effect of the gas is essentiallylost. The gas, therefore, should not be used at or near its saturationpressure. When a single gas is used in a speaker operating at roomtemperature and atmospheric pressure, it should have a normal boilingpoint significantly below room temperature. Higher boiling inventiongases may be used in mixtures provided the partial pressure of eachcomponent gas is significantly below its saturation pressure.

The loudspeakers which are useful in accordance with this invention areelectromagnetic transducers having a sound producing diaphragm. Suitablespeaker systems include direct-radiator, bass reflex, infinite baffleand horn types.

In accordance with this invention, at least one surface of the speakerdiaphragm is in contact with the invention gas. The gas may be incontact with the front side, the back side, or both sides of the speakerdiaphragm. The enclosure retaining the invention gas may be composed ofmetal, wood, plastic or the like. In one embodiment, the gas is retainedin the speaker enclosure by the speaker diaphragm which may be made ofor coated with a material impervious to the gas thus completing theenclosure of the gas. In another design, a continuation 4 of the speakerdiaphragm may be used to serve as a baglike enclosure around the back ofthe speaker, the front of the speaker, or both. The best improvement inquality of sound is obtained when both the front and back surfaces ofthe speaker diaphragm are in contact with the invention gas. For thispurpose, a thin gas-impervious film is stretched across the front of theotherwise airtight speaker enclosure to retain the gas and to transmitsound to the outside air. A film especially suitable for this purpose isa polyester or metallized polyester film. Films made of polyethyleneterephthalate have been found to be particularly suitable. Attenuationof sound intensity by the thin film is readily offset by slightlyincreasing the power input to the speaker.

In retaining the gas of this invention, if a film is used which allowsair to diffuse in and mix with the invention gas, there will be aresultant rise of pressure within the enclosure. As diffusion takesplace, the tension on the film increases which is desirable so long asno rupture of the film or of the enclosure occurs. Diffusion of air intothe enclosure will not interfere with the proper functioning of theinvention gas so long as the partial pressure of the invention gas isunchanged. This inward flow of air may be prevented, of course, byemploying a film which is impervious to both air and the invention gas.

The improved sound quality described above was obtained using speakerswhich were designed for use in air. Accordingly, it is contemplated thatstill further improvements might be achieved with speakers specificallydesigned for use in accordance with this invention.

The following examples, illustrating the novel speaker systems of thisinvention, are given without any intention that the invention be limitedthereto.

EXAMPLE 1 The acoustic property of the gases of this invention is mostreadily observed by placing a portable radio in a box-type enclosurewhich is substantially larger than the radio and is closed on all sidesexcept the top. With the radio tuned to suitable sound such as voice orpreferably music, the air in the box is displaced by passing in theheavier invention gas from a compressed gas cylinder. Immediately thereis a dramatic improvement in the quality of sound from the radio. Theimprovement in bass response is particularly noteworthy even to anuntrained ear.

The following table contains pertinent data for various gases within thescope of this invention. The specific heats, Cp and Cv, were determinedat 1 atmosphere absolute and 21 C.

TABLE Invention Gas Bolling MW Cp/Cv Q P 2 Pt., 0 0. MW 1l,3-bls(trlfluoromethyl)hexafluorocyelobutane 44 300 1. 20Z-ehloroheptafiuoropropane. 2 204. 5 1. 05 0. 51 Chloropentafluoroethane39 154. 5 1. 079 0. 96 Deeafluorobutane 1 238 1. 04 0. 381,2-dichlorohexaflnoropropane..- 34 221 1. 05 0. 551,2-dichlorotetrafiuoroethane. 4 171 1. 078 1. 041,1,1,2,2,3,3-heptafluoropropane. -17 1. 058 0. 57 Hexafiuoroethane 78138 1. 078 0.84 Octafluorocyelobutane -6 200 1. 06 0. 72Octafiuoropropane 38 188 1. 053 0. 531,1,1,2-tetrafiuoroethylheptafiuoropropyl ether 39 237 1. 035 0. 29

When the above gases are individually used to displace air in the boxdescribed above, significant improvements in the quality of sound areobserved in all cases.

EXAMPLE 2 A radio was turned on its back so that the conical speakerdiaphragm faced upward and the diaphragm was filled withl,Z-dichlorotetrafluoroethane from a compressed gas cylinder. A thinpolyethylene terephthalate film was stretched over the face of thediaphragm thereby enclosing the polyfluorinated gas in contact with thefront surface of the diaphragm. When the radio was tuned to suitablesound, an improvement in quality of sound was observed, particularly inthe bass response.

EXAMPLE 3 The following experiments were carried out to determine moreprecisely the nature of the effect of polyfluorinated gas on speakerperformance.

A speaker, system similar to that illustrated in FIGURE 1 was employed.The speaker enclosure 1, which was 12in. x 12 in. x 24 in., was made of/2 in. plywood, glued and screwed together, and was open at each end.The enclosure had an adjustable partition 2 which could be moved toregulate the effective volume of the speaker cavity. The enclosure alsocontained a speaker having a 5 in. diaphragm 3 and a 6.8 oz. magnet 4.The speaker diaphragm was peripherally mounted on a partition 5 whichwas 6 in. from the top of the speaker enclosure and which separated thearea designated 6 behind the diaphragm from the area designated 7 infront of the diaphragm. The enclosure also had a gas inlet 8 forintroducing gas into area 6, and a gas inlet 9 for introducing gas intoarea 7. In some cases, a 0.3 mil polyethylene terephthalate polyesterfilm 10 was drawn tightly over the top'of thei'speaker enclosure toseparate the gas in area 7 from the outside atmosphere.

Signals from a sine-wave generator capable of generating impulses ofconstant energy over a wide range of frequencies: were amplified andsent to the speaker described above. To measure the voltage of thesine-wave generator and verify its constancy, an alternating currentvacuum tube voltmeter was used to monitor the generator. An audioanalyzer was used to monitor the voltage and power level of the signalpassing from the amplifier to the speaker. A sound level meter wasplaced 4 in. above the top of enclosure 1 to measure the intensity ofsound coming from the speaker system and a sound level recorder wasconnected to the meter to record permanently the results in the form ofa frequency response curve.

Tests were made in a room about 10 ft. x 14 ft. which wassemi-soundproofed with thick rugs on all surfaces except the ceiling.The partition in the speaker enclosure was adjusted to provide a cavityof about 1.4 cubic feet below the speaker diaphragm. The speaker wasactivated by signals from the sine-wave generator at frequencies varyingfrom" 50 to 10,000 cycles per second and an energy input to the speakermaintained at about 0.5 watt.

FIGURE 3 illustrates the frequency response curve obtained when areas 6and 7 were filled with air and no polyester film was used. FIGURE 4;isthe frequency response curve obtained when areas Y6 and 7 were filledwith perfiuorocyclobutane and no polyester film was used. FIGURE 5 isthe frequency response curve obtained when areas 6 and 7 were filledwith perfiuorocyclobutane and a polyester film was stretched over thetop of the speaker enclosure.

The improved results obtained in accordance with this invention can 'beobserved by comparing the relative uniformity or flatness of the abovefrequency response curves. The flatness of these curves has beendetermined by computing the standard deviation of each curve from theaverage decibel value for the curve. In FIGURES 3, 4 and 5 the averagedecibel value for each curve is shown as a horizontal line extendingfrom 50 to 10,000 cycles per second. In FIGURE 3, the standard deviationfrom the average decibel value is 4.50 decibels, In FIGURE 4, thestandard deviation is 3.67 decibels, while in FIGURE 5 it is 2.18decibels. In other words, the standard deviation for air is more than1.2 times the standard deviation for perfluorocyclobutane in FIGURE 4and more than 2 times the standard deviation for perfluorocyclobutane inFIGURE 5. This represents a substantial improvement in accordance withthis invention.

The relative flatness of these frequency response curves can also bedetermined by measuring the extent of the frequency range over whicheach of the curves has no more than a 10 decibel variation. Using avariation of no more than 10 decibels as an indication of fairly fiatresponse, the response curve for air in FIGURE 3 is fairly fiat over arange of about 2,000 cycles per second. In the presence ofperfiuorocyclobutane, the response curve in FIGURE 4 is fairly flat overa range of about 5,000 cycles per second, while in FIGURE 5, theresponse curve is fairly fiat over the entire range of about 10,000cycles per second. Thus it can be seen that a significant improvement isobtained in accordance with this invention as represented by FIGURES 4and 5.

In each of the above tests, a record player was connected to theamplifier and musical selections were played through the speaker. Thequality of sound obtained when the speaker diaphragm was in contact withperfiuorocyclobutane was unmistakably more pleasing than that obtainedwhen it was in contact with air. Particularly, the bass and trebleresponse and the clarity of sound were improved.

EXAMPLE 4 Using the procedure and equipment described in Example 3, thenature of the effect of having only the back surface of'the speakerdiaphragm in contact with the gas of this invention was determined asfollows:

The speaker system illustrated in FIGURE 1 was adjusted by means ofpartition 2 to give a speaker cavity of 0.4 cubic foot.Perfiuorocyclobutane was introduced through inlet 8 into area 6. Area 7was filled with air and opened to the atmosphere. The frequency responsecurve obtained is shown as FIGURE 2.

When musical selections were played on the record player through thisspeaker system, the quality of sound obtained was a significantimprovement over that obtained when the speaker diaphragm was in contactwith air on both sides.

Although the invention has been described and exemplified by the use ofparticular polyfluorinated compounds and specific speaker systemdesigns, it is to be understood that it is not limited thereto. As willbe apparent to those skilled in the art, numerous modifications andvariations of these embodiments may be made without departing from thespirit of the invention or the scope of the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A loudspeaker system comprising a loudspeaker containing a soundproducing diaphragm in which at least one surface of the diaphragm is incontact with a gaseous atmosphere of a non-flammable polyfluorinatedaliphatic or alicyclic compounds characterized in that C1) 2 MWQ A) isless than 1.0, wherein MW is the molecular weight of the compound, Cp isits specific heat at constant pressure and Cv is its specific heat atconstant volume.

2. The loudspeaker system of claim 1 in which the gaseous atmospherecontains at least 10% by volume of the polyfluorinated compound.

3. The loudspeaker system of claim 2 in which the polyfluorinatedcompound is octafluorocyclobutane.

4. The loudspeaker system of claim 2 in which the polyfluorinatedcompound is decafluorobutane.

5. The loudspeaker system of claim 2 in which the polyfluorinatedcompound is chloropentafluoroethane.

'6. The loudspeaker system of claim 2 in which the polyfluorinatedcompound is hexafluoroethane.

7. The loudspeaker system of claim 2 in which the back surface of thediaphragm is in contact with the polyfluorinated compound.

8. The loudspeaker system of claim 2 in which the front surface of thediaphragm is in contact with the polyfiuorinated compound.

9. The loudspeaker system of claim 8 in which the polyfiuorinatedcompound is separated from the outside atmosphere by a tightlystretched, thin film which is impervious to the polyfluorinatedcompound.

10. The loudspeaker system of claim 9 in which both surfaces of thediaphragm are in contact with the polyfluorlnated compound.

11. The loudspeaker system of claim 10 in which the thin film iscomposed of polyethylene terephthalate polyester.

References Cited UNITED STATES PATENTS Robbins 181--31 Robbins 181-31Thienhaus 181--3 1 Sullivan 181-31 Spragins et al. 181-31 Reed 18 13 110 STEPHEN J. TOMSKY, Primary Examiner.

1. A LOUDSPEAKER SYSTEM COMPRISING A LOUDSPEAKER CONTAINING A SOUNDPRODUCING DIAPHRAGM IN WHICH AT LEAST ONE SURFACE OF THE DIAPHRAGM IS INCONTACT WITH A GASEOUS ATMOSPHERE OF A NON-FLAMMABLE POLYFLUORINATEDALIPHATIC OR ALICYCLIC COMPOUNDS CHARACTERIZED IN THAT